Spatial light modulators (SLMs) are in wide use in displays systems and are increasingly being used because they offer the benefit of high resolution while consuming lower power and being less bulky than conventional cathode ray tubes (CRTs). One type of SLM display is the digital micro-mirror device (DMD). A DMD “chip” typically has an array of small reflective surfaces (mirrors) located on a semiconductor wafer to which electrical signals are applied to deflect the mirrors and change direction of the reflected light applied to the device. A DMD-based display system image is created by projecting a beam of light to the device, selectively altering the positions of individual micro-mirrors with image data, and directly viewing or projecting onto an image plane, such as a display screen, the image produced by light reflected by the selectively altered mirror positions. Each individual micro-mirror is individually addressable by an electronic signal and makes up one “display element” of the image. These micro-mirrors are often referred to as picture elements or “pixels,” which may or may not correlate directly to the pixels of an image. This use of terminology is typically clear from context, so long as it is understood that more than one pixel of the SLM array may be used to generate a pixel of the displayed image.
Generally, projecting an image from an array of DMD pixels is accomplished by loading memory cells connected to the pixels. Once each memory cell is loaded, the corresponding pixels are reset so that each micro mirror tilts in accordance with the ON or OFF state of the data in the memory cell. Modulating the beam of light with a micro-mirror is used to vary the intensity of the reflected light, such as through pulse-width modulation (PWM).
Pulse-width modulation (PWM) techniques may be used to achieve varying levels of illumination in both black/white and color systems. For generating color images with SLMs, one approach is to use a single DMD and a color wheel having, in its most basic form, filter segments for generating primary colors (red, green and blue, or “RGB”). Data for different colors is sequenced and synchronized to the color wheel so that the eye integrates sequential images into a continuous color image.
More sophisticated color wheels add a neutral-density (ND) filter segment to the color wheel to increase the effective bit-depth of a color (usually green). The ND segment reduces the amount of light that is transmitted relative to the amount of light transmitted by the corresponding non-ND segment filter for the same color (e.g., non-ND green). This allows smaller bits to be created without having to make bit times too small, resulting in an increased bit-depth.
Most lamps used in DMD systems require a maintenance pulse to stabilize their arc and extend their lifetime. Traditionally, such pulses have been hidden in the “spokes” of the color wheel (the transition point from one color-wheel segment to the next). In cases where this lamp pulse is too large to fit within a color-wheel spoke, the pulse may be placed over the ND segment. Placing the pulse over the ND segment increases the amount of light transmitted by the ND segment.
An “effective transmission factor,” which is defined as the product of the ND segment filter transmission and the pulse-to-plateau ratio (PPR) of the pulse over the ND segment, may be used to determine how to compensate for this increase in light. PPR, in turn, is defined as the ratio of the amplitude of the light output during the maintenance pulse to the amplitude of the light output when not pulsed. PWM sequences used in such systems carefully size the bits displayed during the ND portion of the color wheel based on a given effective transmission factor to achieve the desired bitweights. Unfortunately, for many lamps, the plateau drops in amplitude over the life of the lamp, creating an effective increase in PPR. Unless compensated for, severe nonlinearities in the color ramps that use the ND color would result.
Two techniques currently exist for nonlinearity compensation: (1) use compensation Spatial-Temporal Multiplexing (STM) tables to match the increased ND bitweights (see, e.g., U.S. Pat. No. 6,310,591, which issued on Oct. 30, 2001, to Morgan, et al., entitled “Spatial-temporal Multiplexing for High Bit-depth Resolution Displays,” incorporated herein by reference) and/or (2) generate a new sequence that keeps the ND bitweights at the target levels by resizing the bits to take into account the new effective transmission factor.
While both techniques can be used for nonlinearity compensation, the second technique is superior, because it allows target ND bitweights to be maintained. Ideally, a new sequence is written for each effective transmission factor reached during the life of the lamp. However, two practical factors make this difficult. First, a point occurs (at a high enough effective transmission factor) at which creating the target bitweight requires an ND bit time that cannot be achieved because it is less than the absolute minimum bit time for the DMD. The only technique for compensation for effective transmission factors above this threshold is to use compensation spatial-temporal multiplexing (STM) tables (the first compensation technique, above) for the increased bitweights. Second, the amount of memory available limits the number of sequences that can be used for compensation.
Memory requirements become especially large when the system supports multiple color-wheel spin factors (e.g., 2× and 3×), because although the same effective transmission factor range must still be covered, the available memory must be divided between or among sequences for all of the color-wheel rates.
What is needed in the art is a technique for increasing compensation sequence storage density in an SLM system. The technique may be useful for reducing the amount of memory an SLM system requires to provide a certain level of performance or for reducing dither noise that may be associated with the system without requiring more memory for compensation sequences.